1. Field of the Invention
This invention relates to steel cords used as a reinforcement of elastomer articles such as a pneumatic tire, industrial belt and the like as well as pneumatic radial tires using such steel cords. More particularly it relates to steel cords having improved resistance to cord breakage.
2. Description of the Related Art
Steel cords have become widely used for the re-inforcement of various elastomer articles. For example, steel cords formed by twisting 4-5 steel filaments or so-called steel cords of 1.times.4 or 1.times.5 structure are used for the reinforcement of pneumatic tires. Lately, it is strongly demanded to reduce the weight of the tire in order to attain low fuel consumption of a vehicle and the cost of the tire. In order to satisfy this demand, the use steel cords of 1.times.3 or 1.times.2 structure has started.
These steel cords of 1.times.3 or 1.times.2 structure are constituted by merely decreasing the number of steel filaments used, so that when they are used in a belt of the tire, in order to obtain a proper belt strength, it is required to thicken the steel filament used. However, when excessive input is applied to the tire during the cornering of the tire, a so-called buckling deformation is caused in a ground contact portion of the tire located opposite to the cornering direction to buckle the cords used for the reinforcement of the tire. Hence compressive stress is applied to these cords and finally cord breakage occurs. This phenomenon becomes conspicuous in case of using a thick filament.
In order to avoid such a cord breaking phenomenon, it is attempted to add a new reinforcing member before tire build-up, which largely obstructs the reduction of tire weight and cost, so that merits by using steel cords of 1.times.3 or 1.times.2 structure in the reinforcement are considerably lowered.
As to the steel cord of 1.times.3 structure, Japanese Patent Application Publication No. 3-29914 discloses a structure where a steel filament is removed out from the steel cord of 1.times.4 structure to leave the resulting space as it is. Such a cord is considered to exhibit good properties when the compressive stress is applied to the cord. However, the latter cords should maintain the cord arrangement of 1.times.4 structure in form of 1.times.3 structure, but it is very difficult to hold such a form even after the tire building conducted under a high stress such as tire vulcanization or the like. Hence the use of the cords disclosed in this literature is impossible in an industrial scale.
On the other hand, Japanese Patent laid open No. 56-31090 and No. 56-131404 disclose cords of 1+3 structure as a steel cord used as a belt reinforcement for the tire. In these cords, the tensile rigidity is relatively high, but the resistance to cord breakage is poor because buckling easily occurs.